Ions,
You probably can't do what you are trying to do. Someone above said "don't buy a mobo with onboard video". Your predicament is exactly the reason to avoid that advice.
What I have always done is to install the OS using the onboard video and an old, crappy VGA monitor I have lying around. Once about every other driver and chipset installed and working (in other words, one of the last things I do) is install the advanced graphics card. I do this after the OS is completely installed and tweaked the way I like it.
By doing it then, you typically just need to plunk in the AGP (or PCI-E) card, and then hook up a monitor to the new card (whether it supports DVI or VGA, or whatever). By default, and even without drivers installed every advanced graphics card I have used will run in default VGA mode until it's custom drivers are installed.
However, if you plunk in the graphics card and try to run the OS installation from CD, there never seems to be enough native graphics drivers for the installation to start and be seen.
TJR